Mari Lwyd is a welsh tradition, similar to Christmas carolling, where groups of men would carry a horse skull affixed to a pole and decorated with ribbons and a sheet to hide the pole and person carrying it. They would go from house to house, singing and being given food and drink.

I think a stamp of the horse's skull could look really cool and unique. It would also be nice to have a Welsh stamp, since a lot of the current ones are based in England. Perhaps it could be a seasonal Christmas stamp?